Process for removing starch from sweet sorghum juices



United States Patent U.S. Cl. 127-50 1 Claim ABSTRACT OF THE DISCLOSURE Process for removing starch from sorghum juices in which the juices are adjusted to a density of 12-16 Brix, and pH of 7.5-7.8, and are treated with 3-5 p.p.m. of a flocculating agent, such as a high molecular weight polyacrylamide, while at a temperature below 60 C. The resultant juice is concentrated to a density of 30-40 Brix, adjusted to a pH of 7.3-7.7, and treated with l-2 p.p.m. of a flocculant.

A non-exclusive, irrevocable, royalty-free license in the invention herein described, throughout the world for all purposes of the United States Government, with the power to grant sublicenses for such purposes, is hereby granted to the Government of the United States of America.

This is a continuation-in-part of applicaton Ser. No. 592,685, filed Nov. 4, 1966.

Periodic unsuccessful efforts have been made in this country since about 1880 aimed at the commercial production of sucrose from the juices of sweet sorghums. The primary difficulty involved has been the presence of starch in the extracted juices from the sorghum. The starch in the stalks of the sorghum is extracted along with the sugars. The quantity of starch extracted typically varies from about 0.4% to 3.0% of the juice solids, the higher levels being characteristic of the sorghum varieties that provide the juices of highest sucrose content. These latter varieties are, of course, the varieties of most interest from the point of view of sugar production.

Starch is an objectionable impurity in juices intended for sugar production for the reason that conventional processing procedures (i.e., purification and sterilization) require the heating of the juices to approximately 90 C. and at such temperatures starch is gelatinized. The gelatinization of even small amounts of starch will increase the viscosity of the sugar containing syrup to a degree that sucrose crystallization is invariably sharply reduced and often is rendered impossible. Despite the known gelatinization effect on starch of heating and liming, the prior art attempts to produce sugar from sweet sorghum juices employed the liming of the juice to neutrality of even beyond neutrality to alkalinity, and also employed a heat treatment of the juice at temperatures close to the boiling point, after the fashion that is customary for sugarcane juice processing. These unreasonable operations (unreasonable in the case of sorghum juice processing for sugar) provide at least part of the explanation for the unsuccessful attempts to process sorghum juices for sucrose.

Another roblem encountered in prior efforts to process sorghum juice for sugar arose from the attempted use of high-density juices (l620 Brix solids content). These high-density juices were the result of crushing the sorghum stalks using a single pass through the mill rolls. Multiple passes through the mill rolls, with water added at each pass, was considered an unacceptable practice by the sugar "ice processing industry until about the turn of the century even though it was recognized that multiple passes through the crushing rolls raised the overall extraction of sucrose from about 70% to about By this time, serious attempts to process sweet sorghum juices commercially for sugar were no longer being made.

Some efforts were again made to process sweet sorghum juices for sugar in the early l940s, but once more highdensity juices were employed, since it was considered too expensive to concentrate the dilute juices which result from multiple pass crushing operations.

It was proposed, in the early 1940s (see U.S. Patent No. 2,280,085) to remove starch from sweet sorghum juices by the introduction of a centrifugation step just prior to clarification and indeed this mode of operation resulted in the removal of some 70% of starch in the sorghum juice. Conventional liming and heating of the juice as employed in the commercial sugarcane operation could then be relied upon to remove some of the residual starch up to about 93%, provided open-top settling tanks were used, which tanks would allow the removal by skimming of the floating scums that contained starch. The aforementioned patent proposed to centrifuge, clarify by liming and heating, and in addition to make use of a suitable starch-converting enzyme for removal of the last vestiges of starch. The principal objections to such an involved procedure are the excessive time requirements and the expense involved. Sugar processors no longer use batch-type settling tanks for juice clarification and present continuous clarification operations contain no provision for skimming operations. Moreover, the use of enzymes demand sanitary measures quite beyond the average processors operating capabilities. The requisite juice holding times of an hour or more at temperatures which are ideal for the growth and the propagation of sugar-destroying organisms are unattractive from a commercial point of view.

A carbonation procedure similar to that employed in the sugar beet industry might be expected to eliminate starch from sweet sorghum juices with good efficiency, but unfortunately, the temperatures required to obtain adequate calcium carbonate crystal growth are sufficiently high to affect gelatinization of the starch in the sorghum juice. Elevated temperatures and the excess of lime present, together with the large amounts of the invert sugar normally present in the sweet sorghum juices tends to cause excessive color formation. This, of course, is not a problem in the beet sugar industry.

The liming and heating required for clarification and settling of sugarcane juices will cause starch gelatinization in the case of sweet sorghum juices. If lower te peratures of, for example, 25 to 35 C. are resorted to, then inadequate clarification will occur as the result of incomplete coagulation of the protein and phosphate constituents of the juices, and the settling of the juice sediments will require several hours during which interval the sweet sorghum juices are susceptible to fermentation, enzymatic hydrolysis of sucrose, and to dextran formation.

It is the purpose of this invention, therefore, to provide a process for removing, essentially completely, the starch from sweet sorghum juices to the end that the starch-free juices can be processed for sucrose. This process for starch removal has been designed keeping in mind that raw, sweet sorghum juices of low density must be used in the interest of high extraction efiiciences, and that operating conditions must be such that the sugar present in the extracted juices will not be adversely affected either with respect to quality or to quantity.

In general, the process of the invention herein disclosed and claimed, which process for the first time permits of the commercial processing of sweet sorghum juice for sugar, contemplates the utilization of sweet sorghum juices of density not exceeding 16 Brix. These juices are limed to a pH not exceeding 9.0, treated with a flocculant, maintained throughout the process at a temperature which in no case is allowed to exceed 60 C., and permitted to clarify by settling. This clarified juice is then combined with the sugar-containing juices separated from the clarification sediment, limed if necessary to a pH of 7.3 to 7.7, and concentrated to a density of from 20 to 45 Brix in conventional sugar factory evaporation equipment, heated thereafter to from 70 to 90 C., treated with a small quantity of fiocculant, and the syrup allowed to clarify by settling.

More specifically, we can carry out our process using juice densities over the range of 12l6 Brix, which juices have been limed to a pH over the range 7.0 to 9.0, maintained throughout the process at temperatures within the range 15 C. to 70 C., and treated during the processing with an effective amount of a fiocculating agent. The clarified juices are then concentrated to low density syrups of 20-45 Brix. The pH of these syrups are adjusted to the range of 7.4 to 7.7 if necessary, heated to from 70 C. to 90 C., a small quantity of flocculating agent is added, and the syrups allowed to clarify by settling. Still more specifically, we prefer to carry out our processing using sweet sorghum juice densities in the range 13-15 Brix, pH values of the juice in the range 7.5 to 7.8, processing temperatures not exceeding 60 C., and a fiocculating agent in the amount of from 35 parts per million based on the weight of the juice. The clarified juices are then concentrated to low density syrups with densities in the range of 3040 Brix which are processed employing pH values in the range of 7.4 to 7.6, temperatures in the range of 70 C. to 90 C., and a flocculating agent in the amount of 1-2 p.p.m. based on the weight of the syrup.

As mentioned earlier, it is desirable, as is the current practice in the sugarcane and sugarbeet industry, to obtain high levels of sugar extraction from the plant and work with lower density juices. Sucrose extraction levels of 90% usually provide raw juices at about from 12-15 Brix. These lower juice densities favor more rapid sedimentation of suspended material such as starch. In clarifying 12l5 Brix raw juices at neutrality or at pH 8.3 and higher, the sedimentation in the case of sweet sorghum juices has been observed to take place in from l-3 hours. The upper layers, while not entirely clear, are relatively free of starch particles. Sedimentation and compaction of the sedimentary layers is improved under these conditions of Brix and pH, particularly when the juices are heated to about 60 C. before settling. Using the forementioned conditions of juice density, temperature, and pH, and with the addition of a flocculating agent, starch elimination is entirely dependable and occurs in less than 1 hour. The product juices are of greatly improved clarity, some providing clarities of 70% in the Luximeter which instrument is employed for testing sugarcane juice clarities. Sugarcane juice clarities rarely exceed 50% in the Luximeter by way of contrast. In selecting the optimum pH for the raw juice prior to heating and prior to the addition of the fiocculating agent, factors such as ash content and juice color must be considered. It is true that adjusting the pH of the juice to 8.0 or more will improve the clarity of the ultimate juice, but this pH will result in an increased extraction of undesirable red and brown colors. Color apparently arises from finely divided plant material present in the juice. The colors formed are quite refractory to active carbon and to S treatments. Moreover, when liming is continued to the pH of 8.0 or above, soluble lime salts are formed and these salts serve to effectively reduce the juice purity by increasing the mineral content. This situation makes less sugar available for crystallization.

A drop in the pH of the juice occurs when the insoluble rnaterials settle out. Tests show that pH adjustment to 7.8 on the raw juice will yield a clarified juice of pH 7.1 to 7.3. Concentration of this juice to a low density syrup with a density of from 30 to 40 Brix will result in another pH drop, this time to about pH 7.0-7.1. Adjustment of the pH to 7.47.6 before heating this syrup to from 70 C. to 75 C. will provide a settled and clarified syrup of from 7.0 to 7.3 pH, which can then be returned to the evaporator for concentration to a 6070 'Brix syrup suitable for storage. This high density syrup of 6070 Brix will have a pH of about 6.8 to 6.9 following heating, which is considered ideal for the crystallization of maximum quantities of insoluble aconitates.

At a juice clarification pH of 7.5-7.8, there is a minimum of objectionable color formation consistent with the efficient removal of starch and other suspended and colloidal material, and there is a minimum increase in the amount of mineral constituents. Purity rise during clarification of juices from mature stalks generally ranges from 3.5 to more than 5.0. Starch elimination exceeds 97%, which percentage value, under the worst conditions, leaves the final juice with 0.04% of starch based on the juice solids and under the best conditions leaves the final juice with about 0.01% of starch based on the juice solids.

Treatment of the partially concentrated clarified juice as a low density syrup of 30-40 Brix and a pH of 7.4 to 7.6 by heating to 7075 C. and adding a small quantity of flocculant results in a further purity rise of from 0.2 to 0.5 points, at the same time removing approximately 75 to of the starch remaining from the juice clarification. In addition, the sediment which settles out of the hot syrup within a half-hour period, contains significant quantities of insoluble lime salts, phosphates, protein, and other organic non-sugar materials, which coprecipitate with suspended and colloidal matter and thereby provide a syrup of exceptional clarity and low viscosity. At the temperature employed the actions of the plant hydrolytic enzymes and the organisms which would effectively attack the sugars are avoided.

The following examples will set forth in more detail the process of the invention.

EXAMPLE 1 Raw juice in the amount of 16.5 1. was obtained by crushing sweet sorghum stalks. The raw juice measured 17.8 Brix. Juice in the amount of 20.25 1., measuring 14.8 Brix, was obtained by dilution with water. A portion (19.5 1.) of the diluted raw juice was placed in a steel tank (3 feet deep and 10 inches in diameter) provided with an agitator. The temperature of the juice at this time was 22 C. and the pH measured 5.5. A lime solution (Ca(OH) -aqueous) was slowly added to the raw diluted juice with constant agitation until a pH of 7.7 was obtained. At this point, and with continued agitation, ml. of a 0.05% by weight aqueous solution of a fioc culating agent (a high molecular weight polymer of acrylamide) was added. After addition of the flocculating agent, the agitation was stopped and coagulation of the lime-induced fine floc into large clumps /s inch and more in diameter) was instantaneous. The thus treated juice was allowed to settle one hour and the clear upper layer siphoned off and analyzed. The settled material which comprised about 30% by volume of the total juice at this time was freed of the bulk of its juice content and also analyzed.

The analytical data for the juice and settled material are set forth in Table I.

TABLE I TABLE II Clarity Clarified juice Starch (percent Raw Clarified based on transmisjuice Siphoned Filtrate syrup percent sion in Dry of Luxnn- Purity 79. 2 84. 2 84. 0 84. 4 solids Purity solids etcr) Carbonate ash, percent solids 5. 65 5. 70 5. 74 5. 48 Raw juice 14. 74 78. 7 69 Starch, percent solids l. 31 02 0002 0 1 Clarified juice 14. 00 80.7 01 08 Protein, percent solids 2. 84 2. 2!) 2. 34 2. 04 Sediment 21. 16 40. 6 10. 07

The clarified syrup obtained in this experiment was The clarified juice was of a quality suitable for the production of raw sugar since it had been essentially freed of starch and other impurities. At the same time, juice purity was improved.

EXAMPLE 2 11.75 1. of raw juice, 19.4 Brix, and pH 5.13, was obtained by milling sweet sorghum stalks. Sufficient water was added to dilute this juice to 15.2 Brix. A portion of this diluted raw juice (14.7 1.) was measured into an insulated steel tank (26 inches deep and 8 inches in diameter) provided with a slow speed agitator. The temperature of the juice at this time was 24 C. A lime solution (Ca(OH) -aqueous) was added slowly to the raw juice wth constant agitation until a pH of 7.8 was reached. At this point, hot water (approximately 78 C.) was circulated through a heating coil immersed in the limed juice until the juice temperature reached approximately 55 C. The heating coil was then removed from the tank, and 147 ml. of a 0.05% by weight aqueous solution of a flocculating agent (a high molecular weight polymer of acrylamide) was added to the warm limed juice. After addition of the flocculating agent, the agitation was stopped and coagulation of the lime-induced fine floc into large clumps was instantaneous. The thus treated juice was allowed to settle one hour and the clear upper layer siphoned off and analyzed. The coagulated settled material was removed and freed of its sugar content (to a level of approximately 0.8% sucrose on sediment solids) by rapidly dewatering the insoluble solid matter on two paper-covered flat, perforated plate porcelain filter units (78.5 sq. in. filtering surfaces each) under a vacuum controlled at not more than 2 inches of vacuum. No filter aid was employed. Washing of the filter cake was limited to three successive 75 ml. portions of water, and the washings collected were discarded. A sample of the clarified juice filtrate obtained from the filtration of the juice clarification sediment was taken and analyzed. The remainders of the two clarified juice fractions not required for analysis were combined to provide approximately 13.6 1. of clarified juice, which clarified juice was concentrated to provide an approximately 30.5 Brix dilute syrup in a vacuum evaporating unit at a temperature not exceeding 40 C. The pH of this 30.5 Brix syrup was 7.25. 5.65 l. of this syrup was heated in a steel container to approximately 80 C., and 55 ml. of a 0.05% by weight aqueous solution of a flocculating agent (a high molecular weight polymer of acrylamide) was added to the hot syrup while the syrup was being stirred slowly. After addition of the flocculating agent, the agitation was stopped, and the coagulated insoluble material in the syrup formed large clumps immediately and settled rapidly to leave a clear syrup. Settling was complete in less than 30 minutes. with the settled insoluble coagulate occupying less than 10% of the volume of syrup. The thus clarified syrup was decanted free of the insoluble settled material and analyzed. Analytical data for the raw juice, the clarified juice siphoned from the tank, the clarified juice filtrate, and the clarified dilute syrup are set forth in Table II.

of a quality suitable for the production of raw sugar since it had been freed of starch and other impurities. At the same time, juice purity had been improved by 5.2 points, thereby increasing both the quality and the quantity of sugar which could be recovered therefrom.

The following several groups of examples, the data for which in several cases is presented in tabular form, typify the careful effort that went into the design of the process.

They will serve also to display the extreme criticality of All samples were settled 2-5 hours (until upper layers were relatively clear). All samples analyzed less than 0.02% starch based on the juice solids. The color of the clarified juices was increasingly dark as the pH increased. Color formed at pH 9.0 was definitely objectionable.

EFFECT OF ALKALI (TYPE) USED FOR ADJUSTMENT OF JUICE pH Several samples of sorghum juice were diluted to 14.9 Brix density and the adjustment of pH to 7.7 was made with (1) Ca(OH);, (2) Mg(OH) (3) equal parts by weight of Ca(OH) and Mg(OH) and (4) a mixture of equivalent weights of Ca(OH) and Mg(OH) All four samples were treated with 5 p.p.m. of a fiocculating additive and settled for one hour. All four samples exhibited rapid flocculation and settling and yielded juices of satisfactory clarity. Starch contents of the cleared upper layers of juice were (1) .003%, (2) .03%, (3) .02%, and (4) .0l% on the juice solids basis.

Potassium hydroxide and sodium hydroxide are both operable for adjustment of the juice or syrup pH, but sodium hydroxide in particular seems to give rise to objectionable color in the cleared juice layers, especially when the juices have been extracted from stalks which have suffered damage due to borers or other insects which penetrate the stalk. Potassium hydroxide and sodium hydroxide also have the disadvantage of forming soluble salts during neutralization of the juice or syrup acidity, and during reactions with the juice and syrup minerals, so that, in effect, the formation of insoluble phosphates and other insoluble salts during juice or syrup clarifications are not possible and maximum rises in purity during juice and syrup processing are not obtained.

The process which is the subject of this invention is relatively insensitive to temperatures below about 60 C. in treating juices, and between about 60 and C. in processing the low density 30-40" Brix syrups. Juices have been successfully clarified at the temperatures that typically obtain in the laboratory, i.e., 20-30" C., with no effort being made to control temperature precisely. Accordingly, temperatures as low as 20 C. and as high as 60 C. have been encountered with no adverse effect on the process. For optimum clarification, maximum purity rise and starch elimination, a juice temperature of from 50-55 C. is preferred. However, above 60 C. starch included in the sorghum juices starts to gelatinize as noted in the following experiment.

Two samples of sorghum juice, one of 160 Brix density, the other of 14 Brix density, were adjusted to pH 7.7 with lime, treated with a fiocculating additive and allowed to settle. Both juices settled rapidly and provided clear juice upper layer with no detectable starch. A second sample of the 14 Brix juice was given the same treatment except that it was heated to 7075 C. The settled and clarified juice layer in this case contained of the original starch.

EFFECT OF SYRUP TREATMENT ON CLARIFIED JUICES Clarified juice in the amount of 11.5 1. was obtained by clarification of a 15 Brix raw juice at 55 C. and 7.9 pH with addition of 3 p.p.m. of flocculant. Purity rise of 3.8 points was noted, and 16% of the raw juice protein and 97.8% of the starch were eliminated during clarification. This clarified juice was concentrated at low temperature and high vacuum to a low density syrup at 32.8 Brix. The pH was adjusted to 7.45 with milk of lime. Portions of this limed low density syrup were then heated to the temperatures noted below, 2 ppm. of fiocculant added, and the sediments allowed to settle for approximately one-half hour. The analytical data on the clarified juice and the product clarified syrups are given in the following table.

TABLE III Clarified syrups, after heating and settling Clarified juice 70 C. 90 C.

Purity 82. 1 82. 6 82. 4 Carbonate ash, percent solid 7. 47 7.39 7. 47 Starch, percent solids 0121 .005 .003 Protein, percent solids 2. 08 1.93 1. 74 Clarity of 60 Brix syrup (percent transmission in Luximeter) 65 33 36 pH 7. 45 7. 7. 18

These clarified low density syrups contained insignificant quantities of starch, and the protein and carbonate ash contents were decreased slightly. Syrup clarities were excellent and purities had been increased by from 0.3 to 0.5 points through this syrup clarification procedure.

OPERABLE TYPES OF FLOCCULATING ADDITIVES A large number of commercially available fiocculating agents were examined for their ability to clarify sorghum juices and concurrently remove starch. Adjustment of the juice pH to about 7.0 to 9.0 usually provides a finely divided floc that will settle in time and carry down with it a goodly percentage of colored and other suspended matter but in no case, without additive, was the finely divided floc capable of trapping and carrying down as sediment essentially all of the extraneous juice materials (i.e., starch) in a reasonable length of time (e.g., one hour or less) which rate of settling is almost mandatory if juice spoilage is to be avoided.

One fiocculating additive that is eminently satisfactory for the process is of the type referred to commercially as the anionic. This particular material is described as a high molecular weight polymer of acrylamide containing approximately of carboxylic acid groups and 70% of amide groups. The molecular weight of the polymer is said to be someplace between 2 and 3 million and the reaction of the material is anionic in neutral and in alkaline solution.

A second suitable fiocculating additive is of the type referred to as an anionic polyelectrolyte. Yet another additive that has been established as operable in this process is the citrus pectate pulp which material is described by its producers as a high molecular weight pectatecellulose mixture. As for the type of fiocculating additive to be employed in the process, the requirements appear to be indifferent. It is theorized that the anionic type produces clump formation (flocculation) via a dissipation of the electrical charge on the fine floc particles with accompanying co-precipitation of the starch and other suspended matter in the juice. The action of the pectate type would almost certainly be different and may involve the formation of insoluble calcium pectate, magnesium pectate, or iron pectate which precipitates physically enrobe the starch and other suspended matter.

Whatever the specific mechanism involved, the types listed are operable with good and about equal efliciency.

Having thus described the process of the invention, I claim:

1. The process for removing starch granules from raw 0 sorghum juice which consists of:

(a) adjusting the juice density, within the range of about from 12 to 16 Brix;

(b) adjusting the juice to a pH of about from 7.5 to 7.8 and incorporating into the pH-adjusted juice a fiocculating additive in the amount of about from 3 to 5 ppm. while maintaining a temperature which does not exceed 60 C.;

(c) separating the floc from the clarified juice;

(d) concentrating the clarified floc-free sorghum juice to a low density syrup of about from 30 to 40 Brix, and adjusting the pH of this syrup to the range of about from 7.3 to 7.7;

(e) heating this syrup to a temperature within the range of about from 60 to C., and incorporating into this hot syrup a fiocculating additive in the amount of about from 1 to 2 p.p.m.; and

(f) separating the floc from the clarified syrup to pro duce a low density sweet sorghum syrup essentially free of starch granules.

References Cited UNITED STATES PATENTS 1,727,738 9/1929 Van Nes 127-48 X 2,280,085 4/1942 Ventre et al. 127-48 X 2,764,512 9/1956 Wilson 127-49 2,937,143 5/1960 Goren 127-50 X OTHER REFERENCES Bonneville, Clarification Polyelectrolytes, Sugar, November 1953, pp. 36-38.

MORRIS O. WOLK. Primary Examiner D. G. CONLIN, Assistant Examiner US. Cl. X.R. 127-46, 55, 71; 99-142 

